`TagAttribute`s are implemented on top of the low level Attribute API,
and `PersistentEnvExtension`.
This is just the first attribute on a series of attributes we are
going to implement using Lean itself.
We are going to start making drastic changes in the parser,
elaborator, attributes, etc. Examples:
- No View objects. I am going to implement match_syntax.
- No RecT in the parser. I am going to implement parser extensions
using an approach similar to the one I used to implement environment
extensions.
- No Parsec. I will use an approach similar to the one used in the
experiment https://github.com/leanprover/lean4/tree/master/tests/playground/parser
It is easier to perform these changes with the new frontend disabled.
I will slowly re-active it as I apply the changes.
cc @kha
Remark: the attribute actions used by the frontend are all in IO.
These actions access attributes by name, and need access to the IO.ref
that contains all registered attributes in the system.
